This application claims priority to Application No. 10032422.3 which was published in the German language on Jul. 4, 2000.
The invention relates to a method for protecting a transmission path between a base unit and a mobile key unit in an access control system, and in particular for motor vehicles, in which a carrier signal which is modulated with a user signal is transmitted from a transmitting unit to a receiving unit, and to an access authorization control system.
Access authorization control systems are often used for motor vehicle locking systems. They provide a high degree of security and ensure access only to authorized users, while at the same time allowing a high degree of convenience. The latter is of major importance for acceptance of the systems.
Conventionally, such an access authorization control system includes a base unit, which is arranged in a motor vehicle for example, and one or more mobile key units, which are carried by the authorized vehicle users, for example.
Active access control systems have also been known for a relatively long time, in which the user himself carries out some action. The action may be, for example, operation of a pushbutton to initiate the access authorization control process andxe2x80x94if the result is positivexe2x80x94is granted access. Passive access authorization control systems have been developed in order to improve additional user convenience, and can identify that the user is approaching the area to be protected, for example a motor vehicle. Hence, the system can automatically initiate an access authorization control process. In this case, it is for preferable that the checking of access authorization has been completed when the user reaches the protected area to enter, for example when he reaches his vehicle and wishes to open the vehicle door. To this end, the access authorization control system is designed such that the base unit continuously attempts to communicate with, for example, a key unit. When a key unit is identified, the signals are interchanged, and the key unit is authenticated. If the authentication is positive, access is then allowed, for example with the doors of the vehicle being unlocked, an alarm system being deactivated and an immobilizer being disabled. In an ideal situation, the access authorization control system operates so as to be transparent to the user. That is, the user only becomes aware of the resultxe2x80x94namely access to the vehicle. For example, the user is not aware of the system activity, particularly of the data interchange between the base unit and the key unit. However, this convenience, specifically the capability of the entire authentication process to take place without the user being aware of it, carries a certain risk. Unauthorized intervention in the transmission path could occur, and could even possibly be successful, without the authorized user being aware of this.
In order to prevent this, it is necessary to protect the transmission path. The transmission path is generally in the form of a radio path, between the base unit and the key unit. Unauthorized intervention in the transmission path and hence in the communication between the base unit and the key unit should be identified and should be protected against.
In one embodiment of the invention, there is a method for protecting a transmission path between a base unit and a mobile key unit in an access control system. The method includes, for example, modulating a carrier signal with a user signal, transmitting the carrier signal from a transmitting unit to a receiving unit, and transmitting a reference signal, which is at a constant phase angle with respect to the carrier signal, comparing the phases of the reference signal and the carrier signal and emitting an alarm signal if a phase difference which varies with time is detected.
In one aspect of the invention, the reference signal is modulated onto the carrier signal.
In another aspect of the invention, the reference signal is added to the carrier signal at one or more predetermined positions in time.
In another aspect of the invention, the user signal and the reference signal are transmitted in parallel.
In another aspect of the invention, the reference signal is modulated onto the carrier signal by means of I/Q modulation.
In another aspect of the invention, the reference signal is modulated onto the carrier signal by a spread-spectrum method.
In another aspect of the invention, the reference signal is produced from a chip clock for the transmitting unit.
In another aspect of the invention, the clock signal and the reference signal are produced from a common source.
In another aspect of the invention, the receiver carries out demodulation with carrier recovery.
In another aspect of the invention, in order to compare the phases, the phase difference is formed and is checked for its constancy.
In another aspect of the invention, the reference signal is determined during an observation time which can be predetermined.
In another aspect of the invention, the observation time is varied as a function of a security level.
In another embodiment of the invention there is an access authorization control system. The system includes, for example, a base unit and a mobile key unit, each of which has a transmitter and a receiver to transmit a carrier signal, wherein the transmitter in the base unit has a carrier signal source and a reference signal generator for producing and transmitting a reference signal whose phase is locked to the carrier signal, and the receiver has a phase detector and a test circuit, the test circuit configured for evaluating the phase angle between the carrier signal and the reference signal.
In one aspect of the invention, the reference signal generator is a frequency divider and is connected to the carrier signal.
In another aspect of the invention, the transmitter has a modulation unit which modulates the reference signal onto the carrier signal at predetermined times.
In another aspect of the invention, the modulation unit has a plurality of channels to modulate the reference signal and a user signal onto the carrier signal.
In another aspect of the invention, the modulation unit is an in-phase/quadrature modulator, where the user signal is modulated on one channel, and the reference signal is modulated onto another channel.
In another aspect of the invention, the modulation unit is a spread-spectrum modulator.
In another aspect of the invention, the reference signal is supplied to the clock input of a PN sequence generator.
In another aspect of the invention, the receiver has a separate demodulator for the reference signal.
In another aspect of the invention, the transmitter and the reference signal generator, and the receiver and the phase detector, are arranged in the same unit.
In another aspect of the invention, a switching device is provided for the observation time.